1. Field of the Invention
The present invention relates to a method of underground production of gaseous and liquid products from coal and, more particularly, to such a method which utilizes a portion of hot gaseous products from a previous gasification of the coal in a subsequent pyrolysis of the coal.
2. Setting of the Invention
Various methods of underground conversion of coal have been developed and are presently being experimentally utilized. Two of such methods are coal gasification and coal liquefaction. Underground Coal Gasification (UCG) involves pyrolysis of coal and other solid carbonaceous material to produce gaseous products, such as H.sub.2, CO.sub.2 and CO, and char. The char is gasified in the endothermic reaction of carbon with H.sub.2 O or CO.sub.2 at over 1400.degree. F. to produce H.sub.2, CO or CO.sub.2, along with ash. To provide the heat energy necessary to carry out these reactions, an oxidant, such as oxygen or air, can be injected through a wellbore into a coal seam through a wellbore and a combustion zone is initiated in the coal seam which progresses through the coal seam. The combustion zone may move towards the oxidant source as in reverse combustion or may move away from the oxidant source as in forward combustion. The produced gases can thereafter be removed through a separate wellbore. These gases may be used as boiler fuel or transformed into methanol (CH.sub.3 OH) by methods well known in the industry. Coal gasification also produces liquid hydrocarbons, which are highly desirable for their Btu content. However, the amount of liquid hydrocarbons produced by these prior art methods of UCG is small. In underground coal gasification, over 92% of the potential energy in the coal can be recovered at the surface, with combustible gases accounting for about 65% of the total energy produced. However, about 23% of the total recovered energy is in the form of sensible heat of gas and latent heat of vaporization for any steam produced. In the prior art methods, this heat energy from the product gases has not been used and was dissipated.
Direct liquefaction of coal by in situ hydrous pyrolysis is another method of recovering energy from coal and solid carbonaceous material. Hydrous pyrolysis produces gaseous and liquid hydrocarbons in a pyrolysis reaction with coal and water, usually steam, at over 700.degree. F. The liquid hydrocarbons produced are considered high quality because the liquid product is more saturated and paraffinic. In this process, steam is percolated through a coal seam to produce liquid hydrocarbons; however, large quantities of heat energy are required to be injected to heat the coal directly or to heat any water present to produce the steam necessary for the pyrolysis reaction.
There exists a need for the production and recovery of liquid hydrocarbons from conversion of coal and solid carbonaceous material by a method which does not have the heat energy generation requirements of conventional liquefaction of coal, as by using waste energy from another location or process.
One such method for using waste energy is disclosed in U.S. Pat. No. 3,379,248 to Strange. In the patent to Strange, water is injected into a heated formation which is traversed by a combustion zone. The water is heated to produce steam and is recovered at the surface where the heat energy of the produced steam is used to move fluids between the surface and a second portion of the formation. Strange, however, does not disclose recovering gaseous products from a coal gasification process and utilizing the heat energy therein for the liquefaction of coal to produce liquid hydrocarbons.
U.S. Pat. No. 4,057,293 to Garrett discloses a method of liquification of coal wherein pyrolysis is initiated in one portion of a retorting area and oil and gas is withdrawn from another portion, and thereafter the flow of produced gas in the retorting area is reversed to convert any produced char into a gaseous product.
U.S. Pat. No. 4,010,800 to Terry discloses a method of extracting gaseous effluent from a coal bed by performing a gasification process in one coal seam and diverting the hot gases produced therefrom to a second coal seam. The second coal seam is thereby dried and pyrolyzed and resulting gaseous effluents are collected at the surface. Garrett and Terry do not disclose or suggest a method of simultaneously producing liquid and gaseous products from coal and other solid carbonaceous material by liquefying fresh coal with hot gases generated by gasifying another portion of the coal that previously had been liquefied.